cdrom: move shared static to cdrom_device_info

The keeplocked variable in the cdrom driver is shared across multiple
drives, but set in per-device ioctls.  Move it to the per-device struct,
avoiding that the setting on one drive affects the driver's behavior
when closing another.

[ Impact: limit udev's confusion to one drive when a CD burning program
  unlocks the CD door at the end of burning. ]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>